The invention relates generally to methods for annealing semiconductor layers and, more particularly, to methods for annealing semiconductor layers for the manufacture of photovoltaic (PV) devices.
PV (or solar) cells are used for converting solar energy into electrical energy. Thin film PV devices may include a number of semiconductor layers disposed on a transparent support, where one semiconductor layer serves as a window layer, and a second semiconductor layer serves as an absorber layer. Solar radiation travels through the window layer to the absorber layer, where the optical energy is converted to usable electrical energy. Additional layers are often employed to enhance the conversion efficiency of the PV device.
There are a variety of candidate material systems for PV cells, each of which has certain advantages and disadvantages. Cadmium telluride/cadmium sulfide (CdTe/CdS) hetero-junction-based photovoltaic cells are one such example of thin film solar cells.
Thin film solar cell manufacturing typically requires a significant number of processing steps to form multiple layers on a substrate. Because of the large number of processing steps, layers, and interfaces, thin film solar cells can be expensive and time consuming to manufacture. Researchers are continually striving to improve the efficiency and reduce the production costs of the thin film PV cells.
A typical polycrystalline thin film PV cell has a very thin (typically less than 0.2 micron) layer called the “window” layer. The window layer's role is to form a heterojunction in combination with an absorber layer. The window layer desirably is thin enough and has a wide enough bandgap (2.4 eV or more) to transmit most available light through to the absorber layer. For copper indium gallium selenide (CIGS) and CdTe type solar cells, the most common material for the window layer is cadmium sulfide (CdS), which is a direct bandgap semiconductor. Various methods may be used to deposit CdS thin films, for example chemical bath deposition, sol-gel, electrochemical deposition, thermal evaporation, sputtering, sublimation, and spraying.
Typically, a thin layer of transparent conductive oxide (TCO) is deposited between the substrate and the window layer (for example, CdS) to function as a front contact current collector. Cadmium tin oxide (CTO) is one example of a material useful as a TCO in thin film PV cells.
CIGS and CdTe photovoltaic devices can suffer reduced performance due in part to the loss of photons at the TCO layer and/or window layer. Thus, it would be desirable to increase the light transmission of these layers.